mport bpy
from bpy.types import Panel, Operator

class ADDONNAME_PT_main_panel(Panel):
    bl_label = "Main Panel"
    bl_idname = "ADDONNAME_PT_main_panel"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = "New Tab"

    def draw(self, context):
        layout = self.layout

        layout.operator("addonname.myop_operator")


class ADDONNAME_OT_my_op(Operator):
    bl_label = "Button"
    bl_idname = "addonname.myop_operator"

    def execute(self, context):

        return {'FINISHED'}



classes = [ADDONNAME_PT_main_panel, ADDONNAME_OT_my_op]


def register():
    for cls in classes:
        bpy.utils.register_class(cls)


def unregister():
    for cls in classes:
        bpy.utils.unregister_class(cls)


if __name__ == "__main__":
    register()